Why can't you drink when taking antibiotics?

Most antibiotics do not adversely react with alcohol. This is actually an old wives tail dating back to WWII when penicillin was heavily used but in short supply. Soldiers were told that they couldn't drink on the antibiotics in an attempt to keep them sober so that they wouldn't become inebriated and have intercourse with prostitutes, thus catching venereal diseases that would require antibiotic therapy.

However, there are a few antibiotics that do have a true reaction with alcohol. For example, Metronidazole inhibits acetaldehyde dehydrogenase thus interfering with alcohol metabolism. These patients will have side effects if alcohol is consumed while the patient is taking the drug and thus, the patient should be instructed to abstain from alcohol use for the duration of the prescription.